Various methods have been proposed previously for obtaining elemental concentrations in underground formations using neutron irradiation of the formation and measurement of the .gamma. rays resulting therefrom. Examples of such methods are found in U.S. Pat. No. 4,464,569, U.S. Pat. No. 4,810,876 and U.S. Pat. No. 5,097,123 (all incorporated herein by reference). In U.S. Pat. No. 4,464,569 a wireline logging tool incorporating a 14 MeV pulsed neutron source is used to irradiate an underground formation and the resulting capture .gamma. rays from the formation are detected and a spectrum obtained. The spectra are analyzed to obtain relative elemental yields indicative of various elements in the formation. The tool is calibrated against known formation compositions and the relative yields are interpreted as the volume fractions of certain components of the formation. Calibration is required because only a relative measurement is obtained, not an absolute one, there is no removal of borehole or fluid-filled porosity effects and there is no correction for elemental sensitivities. U.S. Pat. No. 4,810,876 describes a method and apparatus for determining elemental concentrations of underground formations in which the formation is irradiated with neutrons and a series of radiation measurements are made. The radiation measurements proposed are natural radiation measurement of K, U and Th, activation .gamma. ray measurement of Al and capture .gamma. ray measurement of a series of other elements. The concentration of Al can be measured in absolute terms by the activation .gamma. ray measurement which allows the other measurements to be corrected. This approach requires two neutron sources, a 252 Cf source for Al activation and a pulsed neutron source for capture spectroscopy. U.S. Pat. No. 5,097,123 proposes the use of a broad energy neutron source, AmBe, for the activation and capture spectroscopy measurements. While this reduces the number of sources, there are still three detectors required together with the associated electronics and so a long, complex and expensive tool results.
It is an object of the present invention to provide a method by which the elemental concentrations of a formation can be obtained which obviates or mitigates at least some of the problems outlined above.
It is another object of the present invention to provide a method of determining such elemental concentrations using only capture spectroscopy measurements.